In the first observing run
(Meinhold & Lubin 1991)
they used an SIS receiver, operating at 91 GHz with a bandwidth
of 550 MHz, and in the second observing run
(Gaier et al 1992)
they used a transistor amplifier operating at 25-35
GHz. This takes advantage of an important new development of the last
five years: the advent of very low noise ``HEMT'' transistor
amplifiers [see e.g.
Das (1987),
Mishra et al (1988),
Chao et al
(1989,
1990) and
Tan et al (1991)]
which has revolutionized
receivers at frequencies between 10 GHz and 50 GHz by providing
receiver noise temperatures at most 50% higher than maser amplifiers
over bandwidths one to two orders of magnitude larger, thus yielding
sensitivities 2-7 times better than the best high frequency maser
amplifiers available in the late 1980's.

In the first run
ten fields separated by an angle on the sky equal to the effective
beamthrow of 1° were observed at constant declination in the
single switching mode. Thus the telescope was pointed at nine
positions. It took 15 minutes for the consecutive observations of the
ten fields. The mean levels for the nine T's were around a
few mK, and there were slow drifts of less than 1 mK per hour. For
each 15 minute scan the mean level and the drift were subtracted from
the data and consecutive scans were then averaged. With the scanning
strategy adopted it would be possible, in principle, to probe angular
scales from 13' to ~ 5°, however the subtraction of the
means and drifts eliminates the information on angular scales larger
than approximately 1°. The results for the first observing run
are shown in
Figure 1d. This data set
has a 2 of 6.9 for 7
degrees of freedom. Meinhold and Lubin carried out Monte Carlo tests
simulating the drifts in the mean level, changing linear drifts with
angle, and Gaussian instrument noise and in each case they recover the
shape of the data set shown in
Figure 1d. Thus the data
correction procedure i was not responsible for the characteristics of
the data set. They then assumed a Gaussian spectrum for the
distribution of sky fluctuations and derived the 95% confidence upper
limit given in Table 3.

These results are particularly
important in placing limits on many models of galaxy formation, for
which the predicted values of T/T peak in the range 10' to
~ 1°. Comparing these results with those of
Readhead et al (1989)at the same power in the likelihood ratio test,
Vittorio & Muciaccia
(1991)
have calculated that the latter results place
slightly more stringent restrictions on cold dark matter cosmologies.
On the other hand
Bond and Myers
(1991a,
b)
have used a Bayesian
analysis which indicates that the former results place slightly more
stringent constraints on cold dark matter cosmologies for the case of
standard recombination, and considerably more stringent constraints
for an b = 0.1
universe with no recombination. These authors
and Bond et al (1991)
have also used the combined South Pole and NCP
data sets to show that hot dark matter models are convincingly ruled
out, and to place interesting constraints on cold dark matter and
isocurvature baryon models with power law density perturbation spectra
having indices of n = -1, n = -0.5, and n = 0.

The most important point to emphasize here is that two completely
independent programs using different instruments at different
frequencies place very similar constraints on the cold dark matter
model, and that modest improvements in the sensitivity of the
observations would either detect intrinsic anisotropy or conflict with
cold dark matter models.

In the second observing run a similar observing strategy was adopted,
and the observations were pushed to considerably higher sensitivities.
The observations were made in four independent frequency channels each
2.5 GHz wide, covering the range 25-35 GHz. The beamwidth was
1°.5 and the chop was 3° on the sky. The data have not yet
been fully analyzed, but the preliminary results are encouraging.
Figure 1e shows the results
for the 32.5-35 GHz channel. If, as
seems likely the clear evidence of ``signals'' in the lower frequency
channels (not shown) is ascribable to Galactic synchrotron emission
which is not evident in the highest frequency channel shown here, then
the data from the highest frequency channel will provide considerably
more stringent limits on intrinsic anisotropy than the observations
discussed above. These multi-channel data also illustrate the problem
which now confronts observers in this field, and which we have already
seen in the cases of the VLA and OVRO observations, namely that the
sensitivities have reached levels at which other sources of cosmic
radiation are a significant source of confusing signals which have to
be subtracted from the data sets in order to study the intrinsic
anisotropy of the microwave background radiation per se.